I like the idea, but there's a huge problem with space or medieval themed city rpgs.
The main problem is that modern buildings are much easier to get right than space or medieval for the majority of players.
What kind of build moderation will there be? I can just imagine that most of the buildings will be boxy and spam a lot of paint fx.
I think you should definitely consider a 'pro builders' area (Like in diggy's metropolis)
Here's some random images I've found online of inspiration which you should concider thinking about (Open in new tab for full res)
Realistically, this is what the 'pro builders' area may look like. I'd have strict build rules so players will have to work hard to create something aesthetically pleasing. I know this may sound harsh but in the long run, the server will look nicer, and the players will be pleased with their creation. Plus a server filled with nice builds will give inspiration to the new clients.